IB DP Biology: Cell structure and microscopy (DP Biology) – Revision Notes

These revision notes condense section A2.2 (Cell structure) of the IB Biology guide, first assessment 2025, for DP Biology. A2.2.1 to A2.2.11 are SL and HL. A2.2.12 to A2.2.14 are additional higher level and are marked HL only. For full explanations and worked examples, read the study guide.

Then test yourself with the practice set, tick off statements on the printable checklist, and see the IB DP Biology hub. A free 10-minute diagnostic shows which topics need most work.

Key definitions

Term	Meaning
Cell theory	Organisms are made of cells; the cell is the smallest unit of life; cells arise from existing cells
Deduction	Using a general theory to predict a specific case
Resolution	How well a microscope separates two close points
Magnification	Image size ÷ actual size
Graticule	Scale in the eyepiece, calibrated with a stage micrometer
Scale bar	A line on an image that stands for a stated real length
Compartmentalization	Division of the cytoplasm by membranes into regions with different conditions
Nucleoid	Region of a prokaryote cell holding its naked DNA
Atypical cell	One whose number of nuclei differs from the usual one
Differentiation (HL only)	Cells becoming specialized through different gene expression

Calculations

To find	Use	Units
Magnification	image size ÷ actual size	none (write ×)
Actual size	image size ÷ magnification	same as image size
Image size	actual size × magnification	same as actual size
Scale bar length	real length × magnification	convert to mm
One graticule division	stage micrometer length ÷ graticule divisions	µm

Conversions: 1 mm = 1000 µm. 1 µm = 1000 nm.

Method in steps

1. Measure image length with a ruler, in mm.
2. Convert actual size to mm (or image size to µm).
3. Apply the triangle once.
4. Check that the answer is a sensible size for a cell or organelle, and write the unit.

Worked reminder

A micrograph has a 15 mm scale bar labelled 0.5 µm. Magnification = 15 mm ÷ 0.5 µm = 15 000 µm ÷ 0.5 µm = ×30 000. A mitochondrion 24 mm wide on the print is 24 ÷ 30 000 mm = 0.0008 mm = 0.8 µm.

Two more reminders

Graticule. Sixty graticule divisions line up with 33 stage micrometer divisions of 10 µm. One division = 330 ÷ 60 = 5.5 µm. A cell 9 divisions long is 9 × 5.5 = 49.5 µm.

Scale bar. A 0.2 µm bar on an image at ×45 000 is 0.2 × 45 000 = 9000 µm = 9 mm long.

Under a different objective lens the graticule value changes, so recalibrate. A scale bar stays correct when an image is enlarged or reduced, but a stated magnification does not.

Developments in microscopy

Technique	Key idea	Benefit or cost
Electron microscopy	Electron beam, much shorter wavelength than light	High resolution, shows ultrastructure; dead, stained specimens, possible artefacts
Freeze fracture	Frozen specimen cracked along the middle of membranes	Shows proteins within the membrane
Cryo-EM	Very rapid freezing; no stain or fixing	Molecules close to natural state
Fluorescent stain	Dye emits light after absorbing light	Stained structure glows
Immunofluorescence	Fluorescent antibody binds a chosen protein	Shows where a protein is located

Structures in all cells

• DNA: stores genetic information.
• Cytoplasm, mostly water: medium for metabolic reactions.
• Plasma membrane of lipids: barrier that separates the cell from its surroundings.

Prokaryote versus eukaryote

Feature	Prokaryote (Gram-positive bacterium)	Eukaryote
DNA	Naked loop in the nucleoid region	Chromosomes of DNA bound to histones, in a nucleus
Nuclear envelope	None	Double membrane with pores
Ribosomes	70S	80S in cytoplasm
Organelles	No membrane-bound organelles	Mitochondria, ER, Golgi, lysosomes and others
Cell wall	Present	Plants and fungi only
Cytoskeleton	Not listed for prokaryotes in the guide	Microtubules and microfilaments

Prokaryote components and their jobs

Component	Job
Cell wall	Supports the cell and resists bursting
Plasma membrane	Controls what enters and leaves
Cytoplasm	Medium for metabolism, holds ribosomes
Naked DNA in a loop	Carries genetic information, lies in the nucleoid region
70S ribosomes	Make proteins

Prokaryote structure varies, but the guide asks only for the Gram-positive model (Bacillus, Staphylococcus).

Eukaryote organelles

Organelle	Function
Nucleus	Holds DNA, controls gene expression
Mitochondrion	ATP production by aerobic respiration
Rough ER	Protein synthesis for secretion
Smooth ER	Lipid synthesis
Golgi apparatus	Modifies, sorts and packages proteins into vesicles
Lysosome	Digests unwanted material
Sap vacuole	Storage and turgor in plants
Cytoskeleton	Support and movement

Processes of life

Remember the eight: reproduction, growth, metabolism, nutrition, excretion, movement, homeostasis and response to stimuli. For a named unicellular organism, give a structure for each. Example: Paramecium uses cilia to move and a contractile vacuole for water balance.

For each eukaryote, the cytoplasm is compartmentalized, so reactions with different conditions run side by side in separate membrane-bound spaces.

Animals, fungi and plants

Feature	Animal	Fungus	Plant
Wall	No	Chitin	Cellulose
Vacuole	Small	Varies	Large central
Plastids	No	No	Yes
Centrioles	Yes	Generally no	No (flowering plants)
Cilia and flagella	Often	Rare	Absent (flowering plants)

Atypical cells

Cell	Nuclei	Link to function
Aseptate hypha	Many, in one cytoplasm	Continuous flow of material
Skeletal muscle fibre	Many	Formed by fusion; long cell
Mature red blood cell (mammal)	None	More room for haemoglobin
Phloem sieve tube element	None when mature	Open channel for sap

Micrograph clues and annotation

• Prokaryote: tiny, no nucleus, nucleoid.
• Plant: wall, large vacuole, chloroplasts.
• Animal: nucleus, no wall; microvilli suggest absorption.
• Drawing: pencil, clean lines, no shading, ruled labels, scale bar.
• Annotating means naming the structure and stating its function.
• Secretory vesicles carry products to the plasma membrane; microvilli increase surface area; the plasma membrane is a thin double line.

HL only: origins and specialization

Endosymbiosis (A2.2.12). Mitochondria arose from an engulfed aerobic prokaryote. Chloroplasts arose later in some eukaryotes in a similar way. Evidence: 70S ribosomes, naked circular DNA, ability to replicate.

Differentiation (A2.2.13). Same genes, different expression, often triggered by the environment.

Multicellularity (A2.2.14). Evolved repeatedly. Advantages: larger body size, cell specialization.

Must-know distinctions

• Prokaryote DNA is naked; eukaryote DNA is bound to histones.
• 70S ribosomes (prokaryotes, mitochondria, chloroplasts) versus 80S (eukaryotic cytoplasm).
• Resolution is not magnification. More magnification does not give more detail if resolution is low.
• Plant cell walls contain cellulose; fungal walls contain chitin.
• Rough ER has ribosomes and makes proteins; smooth ER lacks them and makes lipids.

Quick self-test

1. A cell is 7 µm across. How long is its image at ×4000? Give the answer in mm.
2. An image is 35 mm long at ×5000. What is the actual length in µm?
3. How long is a 2.5 µm scale bar drawn on an image at ×8000?
4. Convert 0.2 mm to µm and 750 nm to µm.
5. Name the three structures found in all cells.
6. State two differences between prokaryote and eukaryote ribosomes or DNA.
7. Give one function of the Golgi apparatus.
8. State two ways a plant cell differs from an animal cell.
9. Explain why a mature mammalian red blood cell can contain more haemoglobin than a nucleated cell.
10. (HL only) State two pieces of evidence for the endosymbiotic origin of mitochondria.
11. A scale bar on a micrograph is 12 mm long and is labelled 0.25 µm. Calculate the magnification.
12. A plant cell measures 90 mm on an image at ×3000. Calculate its actual length in µm.

Answers

1. 7 µm × 4000 = 28 000 µm = 28 mm.
2. 35 mm ÷ 5000 = 0.007 mm = 7 µm.
3. 2.5 µm × 8000 = 20 000 µm = 20 mm.
4. 0.2 mm = 200 µm. 750 nm = 0.75 µm.
5. DNA, cytoplasm, plasma membrane.
6. Prokaryote ribosomes are 70S, eukaryote 80S. Prokaryote DNA is naked and in a loop; eukaryote DNA is bound to histones in a nucleus.
7. It modifies, sorts and packages proteins into vesicles.
8. Plant cells have a cellulose wall, chloroplasts and a large sap vacuole (any two).
9. It has no nucleus, so there is more volume available for haemoglobin.
10. Any two of: 70S ribosomes, naked circular DNA, ability to replicate.
11. 12 mm = 12 000 µm. 12 000 ÷ 0.25 = ×48 000.
12. 90 mm ÷ 3000 = 0.03 mm = 30 µm.

Where marks are usually lost

• Not converting units before applying the magnification triangle.
• Giving a magnification without the multiplication sign, or an actual size without a unit.
• Writing that prokaryotes have no DNA or no ribosomes.
• Listing an organelle without a function when asked to annotate.
• Saying a structure is “in plants” without saying which cell is in the micrograph.
• Confusing resolution with magnification.
• Saying differentiating cells lose genes, rather than switching genes on and off.
• Stating evidence for endosymbiosis without linking it to prokaryotes (HL only).

Official syllabus

This resource is aligned to the IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization, section A2.2 (Cell structure).